CD43 is unarguably one of the most abundant proteins on the T cell surface. However, the literature is full of seemingly contradictory findings on the function of this molecule. In particular, CD43 has been proposed to function as both a co-stimulatory molecule and a negative regulating molecule. These contradictory hypotheses are based on the observation that cross-linking CD43 and CD43-deficiency have the same net effect. in both situations the T cells have augmented responses to stimulus. Second, we find that when T cells interact with APC, CD43 is naturally excluded from the contact site. Together, these data support the hypothesis that, due its abundance and highly glycosylated nature, CD43 acts as a barrier to T cell interactions. We have found that CD43 associates at least 7 phosphorylated proteins, one of which is a key player in the TCR signal transduction. We believe that understanding 1) the mechanism of CD43 modulation, 2) the proteins that move out of the interaction site due to their association with the cytoplasmic tail of CD43 modulation, 2) the proteins that move out of the interaction site due to their association with the cytoplasmic tail of CD43, and 3) the functional consequences of CD43 modulation, will lead to the elucidation of the role this protein plays in immune regulation. Thus, the central hypothesis of this proposal is that the unique ability of CD43 to move away from the T cell: APC interaction site is the mechanism by which CD43 regulates T cell function. Therefore, the goal of this grant is to identify the molecular interactions T cell immune responses. To achieve these goals, we propose the following specific aims: (1) Determine whether CD43 movement occurs through cytoplasmic interactions with the cytoskeleton. We hypothesize that an inside/out signal from the transfected into a CD43-/- T cells to determine the role of highly charged extra-cellular region of CD43 and the intracellular region in the modulation of CD43 from the T cell-APC interaction site. (2) To elucidate the signal transduction pathways involved in CD43 modulation from the T cell-APC interaction site. We hypothesize that T cell receptor signaling leads to the phosphorylation and modulation of CD43 via the association of intracellular protein intermediates. To test this hypothesize that CD43 influences T cell responses by modulating away from the T cell: APC interaction site and that in dong so, CD43 regulates conjugate formation and specifically removes TCR signal transduction molecules from the site of activation. To test this hypothesis, the effect of CD43 modulation on T cell function will be tested by comparing the functional capacity of wild type, CD43-/-, and CD43-/- cells that have been reconstituted with mutant CD43 molecules that have altered modulating abilities.